**5. Glass forming process with "slumping method"**

### **5.1 Effect of molding die surface roughness on formed glass plate surface**

A polished SUS310S precision molding die surface with nanoscale surface roughness cannot be easily obtained. Understanding the relationship between the surface roughness of molding dies and formed glass plates will be effective for cost reduction and manufacturing time reduction. In our previous research, the surface roughness of formed glass plates was smaller than that of the polished molding die

*Precision Polishing Techniques for Metal Molding Dies and Glass Forming Technology… DOI: http://dx.doi.org/10.5772/intechopen.99208*

surface. Therefore, in this work, the effect of the molding die surface on the glass plate surface formed using the slumping process was investigated.

The slumping process was used to form a glass plate using polished SUS310S molding dies with surface roughness of 0.28, 0.13, 0.10, and 0.08 μm. The experimental conditions for the glass forming process were as follows:


**Figure 8.**

*Slumping method with SUS310S molding die for glass. [NHK120-H, 120 min at 670°C in electric furnace]. (a) Overview of glass forming in the electric furnace. (b) Formed glass plate with SUS310S molding die.*

**Figure 8** presents an overview of the glass-forming process in the electric furnace and shows a formed glass plate prepared using the slumping method. As observed in **Figure 8(a)**, one glass plate was placed on a polished molding die surface and then heated to 670°C for 120 min in an electric furnace. The glass plate must be released from the molding die surface with enough cooling time to prevent breakout caused by rapid cooling, as shown in **Figure 8(b)**.

**Figure 9** shows the relationship between the surface roughness of the polished molding die surface and that of the formed glass plate prepared using the slumping method. The surface roughness of the molding dies heated in the electric furnace to form glass plates changed to *Ra* of 0.26, 0.14, 0.11, and 0.13 μm, respectively. After the slumping process, the surfaces of the SUS310S molding die were heated at 670°C, and the metallic mirror surface changed to a blue–black surface, as shown in **Figure 10**. The surface roughness of the glass plate before heating was approximately 0.02 μm in *Ra*; this glass plate had a very smooth surface even though it is a commercial product. The surface roughness before and after heating of the glass plates was almost the same (0.02, 0.02, 0.03, and 0.04 μm in *Ra*, respectively). Each glass plate could be molded by the molding dies with different surface roughness, as

#### **Figure 9.**

*Relationship between molding die surface and formed glass plate surface with slumping method. [NHK120-H, 120 min at 670°C in electric furnace].*

**Figure 10.**

*Used molding die surface at slumping method process. [NHK120-H, 120 min at 670°C in electric furnace].*

*Precision Polishing Techniques for Metal Molding Dies and Glass Forming Technology… DOI: http://dx.doi.org/10.5772/intechopen.99208*

shown in **Figure 9**. These results indicate that the surface roughness of the molding die had a minor effect on the surface of the formed glass plates.

Generally, it is thought that the surface roughness of manufactured products cannot be smaller than the surface roughness of the molding dies. This principle is applied in molding technology in which melted materials are injected into molding dies. However, in the slumping method, a softened glass plate just remolds along the molding die shape. Therefore, it is thought that the surface roughness of the glass plate is not affected by the roughness of the molding die.

### **5.2 Analysis of surface roughness copying mechanism**

**Figure 11** shows schematic view of "slumping method" mechanisms and problems for formed glass plates. The set-up of this process is shown in **Figure 11(a)**, polished molding die have some surface roughness and glass plate of before forming has very smooth surface roughness that is below *Rz* = 10 nm [13]. Glass plate transforms along molding die surface shape by heating at softening temperature. Then transformation of glass plate occurs by only self-weight in this method.

The glass plate deforms at softening temperature, therefore that surface roughness is keep before smooth roughness at a center of deformed glass plate, as shown

**Figure 11.**

*Schematic of "slumping method" mechanisms and problems for formed glass plates. (a) Set up of slumping method. (b) Analysis of glass forming phenomenon. (c) Effect of molding die shape error for formed glass plate.*

in (a) area of **Figure 11(b)**. On the other hand, edge areas of glass plate do not transform along molding die surface shape, because self-weight of glass plate is small in that area as shown in (b) area of **Figure 11(b)** [19, 20]. In this research, real formed glass plate edge areas are almost straight like as a before forming glass plate edge, though that shape is not measured actually.

It was found that the influence of surface roughness of molding die for formed glass plate is considerably small in "slumping method" of this research. However, if molding dies have shape errors, glass plates transform along molding die shape with its errors, as shown in (c) area of **Figure 11(c)**. In this method, shape errors of molding die greatly affects to shape errors of formed glass plate [21–23]. The shape errors and edge transformation of glass plate will be examined as the future works.
